1. Field of the Invention
This invention relates to a roll amount detecting apparatus for detecting the amount of roll of a long footage of web such as film, tape or the like wound on a bobbin such as a reel or the like, namely, the length of the web.
2. Description of the Prior Art
Where the length of a film wound on a bobbin such as a reel or the like is to be detected, a rotatable member has been provided which contacts the bobbin and which is rotatable about a certain fulcrum when the film is wound and which contacts the surface of the film wound on the bobbin, and the angle of rotation of this rotatable member has been detected to thereby detect the amount of roll of the film in an analogous fashion.
In such a roll amount detecting apparatus, however, the dimensional accuracy, mounting accuracy, etc. of the rotatable member have all affected the detection accuracy and it has been impossible to accurately detect the amount of roll and, where this detected amount is to be changed into a digital value, it has been necessary to carry out A-D conversion. Also, to enhance the detection accuracy when the amount of roll has become small, it has been necessary to design the apparatus such that the angle of rotation of the rotatable member when the amount of roll has become small is enlarged, and this has led to a complicated construction of the apparatus.